1. Field of Invention
The invention relates generally to wireless communication, and more particularly to methods and apparatus related to handovers in wireless communication.
2. Description of Related Art
The IEEE Standard for Local and metropolitan area networks, Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems, Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1 dated Feb. 28, 2006 (the “IEEE 802.16e Standard”) prescribed various standards for broadband wireless access systems, and is incorporated by reference herein in its entirety. For example, the IEEE 802.16e Standard prescribed various protocols for handing over a mobile station from an anchor base station to a target base station. The considerations for when a mobile station will effect a handover procedure from a anchor base station to a target base station are outside the scope of the IEEE 802.16e Standard, but generally a mobile station will communicate with an anchor base station for a period of time, and then initiate a handover procedure from the anchor base station to a target base station, following which the mobile station will communicate instead with the target base station when, for example, the mobile station has moved to a location where communication with the target base station is more effective than communication with the anchor base station.
The IEEE 802.16e Standard has some shortcomings. For example, before a mobile station can complete a handover to a target base station, the mobile station requires certain system configuration information (SCI) about the target base station, and the protocols prescribed by the IEEE 802.16e Standard for conveying such system configuration information of the target base station to the mobile station can be cumbersome, and can cause undesirable delays during the handover. Also, a mobile station typically transmits various media access control (MAC) messages' (which may also be referred to simply as “control messages”) to initiate a handover procedure, but the IEEE 802.16e Standard does not include a satisfactory protocol for facilitating reception of such MAC messages at various base stations that may be prospective target base stations for a handover. Further, before a mobile station can complete a handover to a target base station, the mobile station may desirably acquire data representing a distance between the mobile station and the target base station, so that the mobile station can transmit uplink signals at times calculated to cause those signals to be received at the target base station at times that are assigned for uplink signals from the base station, thereby avoiding interference with uplink signals from other mobile stations. However, the protocols prescribed by the IEEE 802.16e Standard for acquiring such data may be cumbersome and can cause unnecessary delays during handover. Still further, the IEEE 802.16e Standard does not prescribe acknowledgement by an anchor base station of an indication by a mobile station that the mobile station will perform a handover from the anchor base station to a target base station. Therefore, if the anchor base station did not receive the indication, or received the indication in error, then the anchor base station may continue attempts to communicate with the base station after the base station has completed a handover to the target base station, which may cause unnecessary interference and congestion. Also, if the anchor base station did not receive the indication, then the anchor base station may not have informed the target base station that the mobile station will perform a handover to the target base station, in which case the target base station may not be prepared to communicate with the mobile station, and a communication interruption may result.